The Terminal app allows UNIX experts to access a UNIX shell and enter commands directly. For those who use this facility frequently, it's not exactly clear how to set the transparency of the Terminal's window. Here's how.
After you launch the Terminal app, found in /Applications/Utilities, go to Preferences under the Terminal's main menu. Then select the Settings tab in the toolbar at the top and also select the Window tab.
Terminal Preferences
Try as you might, you won't find a setting for transparency anywhere until you come here and click on the Background control. That brings up not only a way to set the background color but also set the transparency at the bottom.
Terminal background color has opacity control
That's all there is to it, but in this case, finding is a bit harder than doing.